1. Technical Field
The present invention relates to the semiconductor industry, in particular, to microlithography, and more specifically to a method for synthesizing a holographic photomask used for reconstructing the image recorded on the mask and for subsequently forming the image on the surface of a semiconductor wafer.
2. Description of the Related Art
Lithography and, in particular, photolithography is a well-known technique in semiconductor and printed circuit board (PCB) manufacture for creating electrical components and circuits. Photolithography involves placing a mask in front of a substrate, which has been covered by a layer of photoresist, before exposing both a mask and a substrate to light. The areas of photoresist that are exposed to light react and change chemical properties. The photoresist is then developed in order to remove either the exposed portions of photoresist for a positive resist or the unexposed portions for a negative resist. The pattern formed in the photoresist allows further processing of the substrate, such as, but not limited to, etching, deposition, or implantation.
One method of producing holographic images of integrated circuit (IC) topologies is disclosed in US Patent Application Publication 20110020736 (publication date of Jan. 27, 2011; inventors: Vadim Rakhovskiy, et al). As mentioned in this publication, design of ICs with a characteristic element dimension of 0.1 to 0.01 micron is a major promising direction in current microelectronics development. The high-precision technology (having submicron and micron tolerances) of making precise forms with 3D relief can be used in developing mass production of microrobotic parts, high-resolution elements of diffraction and Fresnel optics, and in other technical fields requiring 3D IC layout of a specified depth and with high resolution of its structures in the functional layer of a device. The latter can be used, for instance, to produce printing plates for banknotes and other securities.
Further progress of up-to-date microelectronics strongly depends on the microlithography process resolution that defines the development level of a majority of current science and technology fields. Microlithography involves coating a solid body (usually a substrate made of a semiconductor material) with a layer of a material sensitive to the used radiant flow, optical radiation, or electron beams. More often, however, a photoresist layer is used to produce an image that corresponds to a specified topology, for example, the topology of a certain layer of the IC being produced. Exposure of the photoresist through a pattern, usually called “a mask”, makes this possible.
The positioning accuracy of the best projection scanning systems (steppers) made by ASML (The Netherlands), which is a leader in the field of microelectronics technology equipment, reaches 10 nm, which is explicitly insufficient for making VLSI ICs with a characteristic element dimension of 20 to 30 nm. The gap between of the steppers' abilities and the industry demand is intrinsic because three to five years are required to develop a stepper for submicron technologies and its cost for mass production, alone, is 10 to 70 million dollars, depending on the resolution required. The cost of development when added to the cost of production amounts to hundreds of millions of US dollars.
At present, photomicrolithography (or photolithography) is widely used in industry. The resolution Δx that it provides is determined by the wavelength λ of the radiation used and the numerical aperture NA of the projection system: Δx=κ1λ/NA (W. Moro “Microlithography”; in 2 parts. Part 1: Transl. from English; Moscow. MIR, 1990, p. 478). Such dependence reasonably encouraged developers to use more and more shorter-wavelength radiation sources and more and more larger-aperture projection systems. As a result, for the last 40 years industrial projection photolithography has switched from using mercury lamps with a characteristic radiation wavelength of 330 to 400 nm to excimer lasers with an operating wavelength of 193 nm and even 157 nm. Projection lenses of modern steppers have reached 600 to 700 mm in diameter, which has caused a rapid increase in stepper cost.
The resolution increase results in a sharp decrease in focusing depth ΔF since ΔF=±λ/2(NA)2 [p. 478]] which causes a reduction in output rate and a drastic complication in the focusing system of giant projection lenses and which, again, means higher costs of steppers. Moreover, the side effects limit using the apertures of such lenses during operation at maximum resolution.
In the development process of projection photolithography, the critical dimension of the projection parts decreased at an average of 30% every two years, thus doubling the quantity of transistors in an IC every 18 months (Moore's Law). Nowadays, “0.065 micron technology” is used in the industry, which makes it possible to print parts with a resolution of 65 nm. According to experts' opinions, the next milestone is the development of projection systems and radiation sources providing reliable resolution at a level of 22 nm. Currently, the successful development of projection lithography DUVL enabled to reach through <<multi patterning>> technology a resolution of 14 nm, whereas a switch to extreme ultraviolet (EUV) sources or even to soft X-ray radiation, will not be able to reach a goal of serial CD production until 2022. At present, only experiments with λ=13.4 nm microlithography devices are being conducted. The first such device, as announced at the Intel Developer Forum (Intel™ is the world leader in VLSI IC production), already had been created and in 2002 it was used to produce transistors with a characteristic dimension of 50 nm. However, experts believe that the cost of such a stepper, even in case of its volume production, would reach 125 million USD, and, according to most optimistic estimates, five to ten years will be required to master the technology of mass production of microprocessors having critical dimensions at a level of 20 nm.
One of the most critical constraints of photolithography application is related to diffraction from the edges of a mask (diffraction from edges of the screen) used to attain a desired projecting image on a photoresist surface. As the monochromatism of used radiation increases, the above-stated effect deteriorates the quality of the received image due to occurrence of diffraction maximums placed at distances of the A order from the center of a projected line. If one takes into account that leading manufacturers currently use laser radiation with a wavelength λ=193 nm and even less (in experimental steppers), the significance of the resolution constraint caused by diffraction on the mask edges becomes clear.
Thus, existing projection devices designed to generate images on a light-sensitive layer have a number of essential drawbacks, as follows.
1. Fundamental difficulties of combining high resolution and considerable depth of focus in one device.
2. Considerable complication in the design and technology of projection devices as the wavelength of radiation used to project an image onto a photoresist becomes shorter.
3. Drastic complication in the optical system and technology of producing a projected object (a mask) as the wavelength used for projection becomes shorter.
4. Significant increase in technological requirements and equipment prices as the integration scale in the manufactured products grows.
5. Extremely low technological flexibility in the production process and very high cost of its modification.
6. Unfeasibility in the principle of diversified manufacturing, i.e., fabrication of various ICs on the same substrate during a common technological process.
There is a method of producing a binary hologram by generating a plurality of transmission areas at specified locations or earlier calculated positions on a film. The hologram is opaque to the used radiation in such a way that when illuminated, these transmission areas make it possible to produce a holographic image at a predetermined distance from these areas (L. M. Soroko, “The Fundamentals of Holography and Coherent Optics”; Moscow, Nauka, 1971, pp. 420-434). This monograph considers the possibility of producing a “numeric” hologram, also called a “synthetic”, “artificial”, or “binary” hologram, and sets forth the theory with the conciseness and clarity peculiar to mathematic descriptions. However, the known method of making binary holograms—wherein the image of the transmission areas is produced, for example, by graphical means and then photographed with a significant reduction—does not provide a desired image quality and high resolution primarily because of insufficient accuracy in its production and an insufficient number of transmission areas.
There is a method for producing an image on material that is sensitive to used radiation by a hologram. In this method, exposure spots are generated by imaging at least one hologram placed in front of the radiation-sensitive material (GB 1331076 A, publ. Sep. 19, 1973[3]). However, the known method of using a hologram to provide an image on the material that is sensitive to used radiation does not allow production of high-quality images due to mutual overlapping of a plurality of diffraction orders, and due to the impossibility of using short-wave radiation sources. Moreover, the main objective of this method was to provide effective control of visually checked marks.
Also known is Russian Patent RU2396584 issued on Aug. 10, 2010 to M. Borisov, et al (equivalent to US Patent Application Publication 2011/0020736) which relates to a method for creating holographic images of drawings, wherein an image of the initial drawing is converted into a digital raster image. The diffraction pattern on each point of the future hologram is calculated, where the said diffraction pattern is created from all emitter elements of the digital raster image. Next to be calculated is the interference pattern obtained from interaction of the calculated diffraction pattern with the calculated wave front from a virtual reference point or extended radiation source, which is identical to the real wave front of the source and which will be used in producing the holographic image of the drawing. The result is used as a signal for modulating the radiation beam, which forms the diffraction structure of the hologram on a carrier. The hologram is composed of a set of discrete elements distinguished by their optical properties.
The apparatus for patterning a workpiece using an in-line holographic mask (ILHM) is disclosed in U.S. Pat. No. 5,015,049 issued to Byung J. Chang on May 14, 1991. This patent discloses a method of forming holographic optical elements free of secondary fringes. Holographic optical elements relatively free of unwanted, secondary fringes are produced by passing the light beam from a laser through a rotating diffusing plate to generate a beam of light having a very limited coherence length and a spatial coherence that changes over time. A photographic emulsion having a mirror supported on its reverse side is illuminated by the beam, and interference occurs between this primary illumination and illumination reflected from the mirror, thus creating fringes. No other interference fringes are formed because of the lack of coherence between secondary reflections and other rays of the incident beam. The rotation of the diffusion plate time-averages to zero any random interferences, thus eliminating the speckle pattern. Alternatively, the illuminating beam has a high degree of spatial coherence but its temporal coherence is reduced and varied over a period of time by changing the wavelength of a tunable-dye laser.
U.S. Pat. No. 6,618,174 issued on Sep. 9, 2003 to William P. Parker, et al, discloses an optically made, high-efficiency in-line holographic mask (ILHM) for in-line holographic patterning of a workpiece and apparatus and methods for performing same. The ILHM combines the imaging function of a lens with the transmission properties of a standard amplitude mask, obviating the need for expensive projection optics. The ILHM is either a type I (nonopaque) or type II (opaque) specialized object mask having one or more substantially transparent elements that can be phase-altering, scattering, refracting, and/or diffracting. A method of creating a pattern on a workpiece includes the steps of disposing an ILHM, disposing a workpiece adjacent to the ILHM and illuminating the ILHM to impart a pattern to the workpiece. In another method, the ILHM is used in combination with a lens. The ILHM is disposed such that a holographic real image is formed at or near the lens object plane, and the workpiece is disposed at or near the lens image plane.
U.S. Pat. No. 7,312,02 issued on Dec. 25, 2007 to Shih-Ming Chang discloses a hologram reticle and method of patterning a target. A layout pattern for an image to be transferred to a target is converted into a holographic representation of the image. A hologram reticle is manufactured that includes the holographic representation. The hologram reticle is then used to pattern the target. Three-dimensional patterns may be formed in a photoresist layer of the target in a single patterning step. These three-dimensional patterns may be filled to form three-dimensional structures. The holographic representation of the image may also be transferred to a top photoresist layer of a top surface imaging (TSI) semiconductor device, either directly or using the hologram reticle. The top photoresist layer may then be used to pattern an underlying photoresist layer with the image. The lower photoresist layer is used to pattern a material layer of the device.
A method of generating a holographic diffraction pattern and a holographic lithography system are disclosed also in US Patent Application Publication 2008/0094674 (published on Apr. 24, 2008; inventors are Alan Purvis, et al). The method involves defining at least one geometrical shape; generating at least one line segment to represent the at least one geometrical shape; calculating a line diffraction pattern on a hologram plane, including calculating the Fresnel diffraction equation for an impulse representing the at least one line segment with a line width control term and a line length control term; and adding vectorially, where there are two or more line segments, the line diffraction patterns to form the holographic diffraction pattern. The method and system enables holographic masks to be generated without creating a physical object to record. The required shapes or patterns are defined in terms of a three-dimensional coordinate space, and a holographic pattern is generated at a defined distance from the shapes in the coordinate space.
U.S. Pat. No. 7,722,997 issued on May 25, 2010 to Shih-Ming Chang, et al, discloses a hologram reticle and method of patterning a target. A layout pattern for an image to be transferred to a target is converted into a holographic representation of the image. A hologram reticle is manufactured that includes the holographic representation. The hologram reticle is then used to pattern the target. Three-dimensional patterns may be formed in a photoresist layer of the target in a single patterning step. These three-dimensional patterns may be filled to form three-dimensional structures. The holographic representation of the image may also be transferred to a top photoresist layer of a top surface imaging (TSI) semiconductor device, either directly or using the hologram reticle. The top photoresist layer may then be used to pattern an underlying photoresist layer with the image. The lower photoresist layer is used to pattern a material layer of the device.